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Abstract

The 1.07 Myr old Bosumtwi impact structure (Ghana), excavated in 2.1–2.2 Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U–Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145 ± 82 Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb–Sr and Sm–Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb–Sr and Sm–Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n = 29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846 ± 160 Ma, with an initial Sr isotope ratio of 0.703 ± 0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible “metamorphic age” of ~ 1.8–1.9 Ga tentatively derived from our U–Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb–Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.

1. Introduction

The Bosumtwi crater, centered at 06°30´N, 01°25´W, in Ghana (West Africa; Fig. 1), is a well-preserved complex impact structure with a rim-to-rim diameter of ~ 10.5 km (Koeberl et al., 1998; Koeberl and Reimold, 2005) and a small central uplift (e.g., Scholz et al., 2002). The shock-induced metamorphism that affected the rocks from the Bosumtwi area, has been investigated in detail during the last years (e.g., Boamah and Koeberl, 2006; Coney et al., 2007a; Ferrière et al., 2007a, 2008; Morrow, 2007). It is clearly established that this shock metamorphic event took place about 1.07 Myr ago (Koeberl et al., 1997); however, the precise primary age of the Bosumtwi country rocks, and possible other metamorphic event(s) recorded by these rocks, have not been studied in detail. The 2004 International Continental Scientific Drilling Program (ICDP), during which two impactite cores were recovered, from the deep crater moat (LB-07A) and the outer flank of the central uplift (LB-08A; see Koeberl et al., 2007 for review), gave the unique opportunity to further constrain the parent material of both, the proximal impactites (mainly suevite) and the distal impactites (i.e., the Ivory Coast tektites). For this purpose, U–Pb ages of single detrial zircon grains from Bosumtwi impactites were measured, assuming that zircons have retained their primary (i.e., pre-impact) geochemical and isotopic characteristics. This assumption is realistic in the present case considering that no shock-induced features were observed in any of the measured zircons grains. In addition, analyses of Sr and Nd isotopic compositions of crater-fill impact breccia and shocked basement rock samples were obtained and compared with isotopic data previously determined for samples from outside the crater rim.

The Bosumtwi crater was excavated in lower greenschist-facies supracrustal rocks of the Birimian Supergroup (Wright et al., 1985; Leube et al., 1990; Roddaz et al., 2007) and is the likely source crater for the Ivory Coast tektites (e.g., Gentner et al. 1964; Jones 1985; Koeberl et al. 1997, 1998). The Birimian Supergroup is an assemblage of metasedimentary rocks (dominant) and volcanics that are now altered and metamorphosed to greenstones (Wright et al., 1985; Leube et al., 1990; Roddaz et al., 2007). These greenstones, mainly hornblende-actinolite schist, calcite-chlorite schist, mica schist, and amphibolites, result of the metamorphosed basalts and andesites, whereas the metasedimentary rocks, mainly phyllites, meta-graywackes, quartzitic meta-graywackes, shales, and slates, result of the metamorphosed turbiditic wackes, argillitic rocks, and volcaniclastic rocks (see e.g., Wright et al., 1985; Leube et al., 1990; Roddaz et al., 2007). The supracrustal rocks of the Birimian Supergroup are present in Ghana in the form of parallel volcanic belts, several hundred kilometres in length, and separated by basins filled with dacitic volcaniclastics, wackes, argillitic sediments, and granitoids (e.g., Wright et al., 1985; Leube et al., 1990). An age of 2.0–2.3 Gyr was determined using Sm–Nd isotopic data for these Birimian supracrustal rocks of western Ghana (Taylor et al., 1992). Other geochronological data for rocks from the West African Craton in Ghana show that magmatism in this part of the globe occurred during a period between 2.1 and 2.2 Ga (see Leube et al., 1990; Hirdes et al., 1992). According to the Paleoproterozoic evolution model of Feybesse et al. (2006), an extensive monzonitic magmatism event occurred around 2.16–2.15 Ga, forming the first segments of continental crust in the Ghanaian province.

Proterozoic granitic intrusions, weathered granitoid dikes, and dolerite and amphibolite dikes are also present close to the crater (see Junner 1937; Woodfield 1966; Moon and Mason 1967; Reimold et al. 1998; Koeberl and Reimold, 2005; Fig. 1), and were intruded syn-orogenically or late-orogenically with the folding of the basins, subsequently to the termination of the volcanic activity in the region (Leube et al., 1990). On the basis of U–Pb zircon and monazite dating, Hirdes et al. (1992) determined thatsimilar intrusions, the so-called Belt-type granitoids in the Ashanti belt are 2172 ± 2 Myr old, and that the Kumasi Basin-type granitoids are 2116 ± 2 Myr old. However, no one of the granitoid intrusions at or near the Bosumtwi crater has yet been dated precisely.

Rb, Sr, Sm, and Nd abundances, and Sr and Nd isotopic ratios of impactites and country rocks from the Bosumtwi impact structure, compared to those of Ivory Coast tektites.

3. Sample descriptions

The twelve samples investigated in this study are all from the Bosumtwi impact structure: eleven of them are from drill core LB-08A (see Ferrière et al., 2007a), and one sample, suevite LB-44, is from outside the northern crater rim (collected by one of us (C.K.) at 6°33.88′N/1°23.88′W). For the single-zircon U–Pb dating, one suevite (sample KR8-004) and two meta-graywacke (samples KR8-032 and KR8-109) were selected, because a few zircon grains were noted in these specific samples during petrographic observations (Ferrière et al., 2007a), and with the purpose to investigate a sample from the top of the basement section (i.e., sample KR8-032; depth = 274.99 m below lake level [bll]) and one sample from the bottom of the section (i.e., KR8-109; depth = 425.24 m bll). For the whole-rock Rb–Sr and Sm–Nd isotopic compositions, six suevite samples, 5 from inside the crater (KR8-001, KR8-005, KR8-026, KR8-042, and CAN-31) and the LB-44 sample from outside the crater rim, three meta-graywacke (KR8-032, KR8-066, and KR8-109), and two phyllite samples (KR8-002 and KR8-084) from core LB-08A, were analyzed.

Suevite samples (i.e., polymict impact breccia that includes melt particles; see Stöffler and Grieve, 2007) have a grayish, fine-grained fragmental matrix and consist of rock and mineral clasts and of secondary minerals, mainly smectite, chlorite, and calcite, in the form of very fine-grained aggregates or micro-veinlets. Meta-graywacke and phyllite form the dominant lithic clasts, whereas quartz is the main mineral clast (see, e.g., Ferrière et al., 2007a; Deutsch et al., 2007).

The investigated meta-graywacke samples are light to dark gray in color, medium-grained to gritty, and are mainly composed of (in order of decreasing abundance): quartz, feldspar, muscovite, chlorite/biotite, calcite, and accessory minerals, such as epidote, pyrite, sphene, apatite, zircon, rutile, and allanite. In most samples, biotite is altered to chlorite, and microfractures filled with iron oxides occur.

Phyllite samples are greenish to dark gray in color, very fine-grained, well-banded, folded, and mainly composed of mica (muscovite and sericite) and quartz, with variable amounts of feldspar, chlorite, biotite, rutile, sphene, and pyrite. Minor fracturing occurs in the samples, with some of the microfractures filled with iron oxides. Sample KR8-002 displays numerous quartzitic laminae/ribbon quartz. Detailed petrographic descriptions for each of the samples, as well as the depth of recovery, are given in Ferrière et al. (2007a).

4. Sample preparation and experimental methods

Single zircon U–Pb dating were performed at the State Key Laboratory of Continental Dynamics (Northwest University, China), using a laser ablation inductively coupled plasma spectrometer (LA-ICP-MS). The three impactites samples were crushed, sieved, and heavy minerals were concentrated using methylene iodide. Zircons to be dated were hand-picked under a binocular microscope, mounted in epoxy, and polished. All mounted grains (about 140 in total) were documented using cathodoluminescence (CL), prior to measurements, to identify pristine areas for analysis and to determine if multiple age components (i.e., core and overgrowths) occur (see Fig. 2). A total of 31 laser spot analyses in 21 zircon grains were performed. The spot size and frequency were 30 µm and 10 Hz, respectively. Each spot analysis consisted of about 30 s background acquisition and 40 s sample data acquisition. Five to seven sample analyses were followed by measurement of three international standards; Harvard zircon 91500, NIST SRM 610, and Australian Macquarie University zircon GJ1. The isotopic ratios 207Pb/206Pb, 206Pb/238U, and 207Pb/235U were calculated using GLITTER 4.0 software (Macquarie University, Australia) and corrected using Harvard zircon 91500 as external standard. For the evaluation of U–Pb isotopes of Harvard zircon 91500, the Australian Macquarie University zircon GJ1 was used as external standard. Analytical techniques, standards, instrumentation, and data correction are described in detail by Liu et al. (2007). Ages were calculated using Isoplot 3 (Ludwig, 2002).

Cathodoluminescence (CL) images of the analyzed zircon grains from suevite (KR8-004) and meta-graywacke (KR8-032 and KR8-109) samples from the LB-08A drill core in the Bosumtwi crater. The circles locate the U–Pb laser ablation pit, 30 μm...

The cathodoluminescence images of the zircon grains were obtained with an Oxford Mono-CL system attached to a JEOL JSM 6400 SEM at the Department of Mineralogy and Petrography, Natural History Museum, Vienna (Austria). The operating conditions for the CL investigations were 15 kV accelerating voltage, 1.2 nA beam current, and monochromator grating with 1200 lines/mm.

Whole-rock Sr and Nd isotope analyses were performed at the Department of Lithospheric Research (University of Vienna) using a ThermoFinnigan Triton TI thermal ionization mass spectrometer (TIMS). About 100 mg of the powdered bulk rock samples were dissolved to allow chemical separation of the elements (Sr, Nd). Sample digestion was performed in tightly closed Savillex® beakers, using a 5:1 mixture of ultrapure HF and HClO4, for 3–4 weeks at ~ 100 °C on a hot plate. Then, after acid evaporation, repeated treatment of the residue using 5.8 N HCl resulted in clear solutions. The Sr and REE fractions were extracted using AG®50 W-X8 (200–400 mesh, Bio-Rad) resin and 2.5/4.0 N HCl. Nd was separated from the REE fraction using teflon-coated HdEHP, and 0.18 N HCl, as elution media. Maximum total procedural blanks were < 100 pg for Nd, and < 1 ng for Sr. After overnight drying, Sr and Nd IC (isotope composition) samples were loaded on a Re filament using 1 N HNO3, and ionized using the Re double filament evaporation technique and the ThermoFinnigan Triton TI instrument.

A 87Sr/86Sr ratio of 0.710253 ± 0.000006 (n = 4) was determined for the NBS987 (Sr) and a 143Nd/144Nd ratio of 0.511844 ± 0.000003 (n = 4) for the La Jolla (Nd) international standards, respectively, during the period of our investigations. Within-run mass fractionation was corrected for 86Sr/88Sr = 0.1194 (Sr) and 146Nd/144Nd = 0.7219 (Nd), respectively. Uncertainties on the Sr and Nd isotope ratios are quoted as 2σm. Errors on the 147Sm/144Nd ratio are given as ± 5.0%, representing maximum errors; regression calculation is based on these uncertainties. Age errors are given at the 2σ level. Isotopic ratios are expressed in ε notation where εNd is the measured deviation in parts of 10− 4 of the 143Nd/144Nd ratio from the present-day chondritic uniform reservoir (CHUR) value of 0.512638 and εSr is the measured deviation in parts of 10− 4 of the 87Sr/86Sr ratio from the unfractionated mantle reservoir (UR) reference of 0.7045 (e.g., Faure, 1986).

The elemental concentrations of Rb, Sm, and Nd were determined by instrumental neutron activation analysis (INAA) at the Department of Geological Sciences, University of Vienna (Austria), whereas the Sr abundances were determined by standard X-ray fluorescence (XRF) spectrometry at the University of the Witwatersrand, Johannesburg (South Africa; see Ferrière et al., 2007b, 2010).

5. Results and discussion

The investigated zircon grains from the Bosumtwi impactites have dimensions between about 100 and 300 µm. Some of them display complex internal structures when observed with cathodoluminescence, such as re-crystallized domains (e.g., SA-52; see Fig. 2) or magmatic zoning with small rims of metamorphic overgrowth around an inherited core/grain (e.g., SA-31). Several zircon grains display also inclusions (e.g., LC-06). However, no specific shock-induced features (such as amorphous lamellae; see e.g., Wittmann et al., 2006) were observed in any of the investigated zircons grains. The CL images of all 21 analyzed grains are presented in Fig. 2. The 207Pb/206Pb age (the more precise age for old zircons; i.e., from the Proterozoic) for each laser ablation pit is also indicated on each CL image. The obtained isotope ratios and U–Pb ages are reported in Table 2. Concordia diagrams of the U–Pb isotope data are presented in Fig. 3 and show discordant ages for the investigated zircon grains. The 207Pb/206Pb apparent ages are dispersed over the range ~ 2.38 to ~ 1.82 Ga, with a cluster of ages around 2.1–2.2 Ga. Only three zircon grains show an apparent age less than 2.0 Ga: the laser spot analyses SA-52c and SA-67b (zircons from the meta-graywacke sample KR8-109), corresponding to a re-crystallized domain and to a metamorphic overgrowth, respectively, and SB-04, the only zircon from suevite that was analyzed in this study (see Figs. 2 and 3). However, it cannot be totally excluded that the SB-04 zircon grain analyzed was somewhat affected by the high pressures (and temperatures) involved in the formation of suevite, even though the grain shows no apparent shock-induced features (see Fig. 2). The successive zircon growths and re-crystallized domains, as revealed with CL, might be partly responsible for the observed discordant ages. Using only analyses that are less than 4% discordant, an upper Concordia intercept age of 2145 ± 82 Ma is obtained (Fig. 3b). This crystallization or magmatic age is in very good agreement with previous geochronological data for rocks from the West African Craton in Ghana, which showed that magmatism in this region occurred in the period between 2.1 and 2.2 Ga (e.g., Leube et al., 1990; Hirdes et al., 1992; Feybesse et al., 2006). Thus, it is likely that the source rock that form part of the metasediments that were excavated by the Bosumtwi impact event originated during this magmatic event, which led to the formation of the first segments of continental crust in the Ghanaian province, as discussed by Feybesse et al. (2006).

U–Pb Concordia diagrams of zircon grains from Bosumtwi impactites. Error ellipses are 2σ. All measured zircons are plotted in (a), whereas only analyses with less than 5% discordance are plotted in (b). Dashed lines (1) and (2) reported...

Isotope ratios and U–Pb ages of zircons from suevite (KR8-004) and meta-graywacke (KR8-032 and KR8-109) samples from the LB-08A drill core in the Bosumtwi crater (Ghana).

Based on our U–Pb isotope data, a possible metamorphic age of ~ 1.8–1.9 Ga can be tentatively derived from the Concordia diagram shown in Fig. 3a. A different inland contribution could be also responsible for the observed trend, however, no magmatic event is known in this region at this period of time.

In addition to the U–Pb dating of zircon grains, whole-rock Rb–Sr and Sm–Nd isotopic analyses of suevite and Bosumtwi basement rocks were performed to provide further insights into the timing of the metamorphism of the Bosumtwi basement rocks, and to better constrain the parent material of the suevite and Ivory Coast tektites. The results obtained for the 11 investigated samples (i.e., six suevite, three meta-graywacke, and two phyllite) are presented in Table 1 and compared with previously published Sr and Nd isotopic compositions of country rocks from the Bosumtwi, suevite from outside the crater rim, and Ivory Coast tektites (Shaw and Wasserburg, 1982; Koeberl et al., 1998).

Our country rock analyses are quite comparable to previous data from Shaw and Wasserburg (1982) and Koeberl et al. (1998). In addition, we report the first 87Sr/86Sr and 87Rb/86Sr ratios for suevite samples (see Fig. 4). These suevite data show very limited spread in Rb/Sr and plot just between the data for meta-graywacke and Ivory Coast tektites (Fig. 4). As already mentioned by Koeberl et al. (1998), it is obvious that 87Sr/86Sr and 87Rb/86Sr ratios vary considerably between the different Bosumtwi country rocks (Fig. 4). For instance, large differences are observed between the phyllite samples, with 87Sr/86Sr ranging from 0.714000 to 0.750069 and 87Rb/86Sr ranging from 0.454 to 1.893. These differences are possibly the result of some modal composition differences between the investigated phyllite samples, such as in the content of Rb- and/or Sr-bearing phases. On the other hand, very limited differences occur among the meta-graywacke samples, with 87Sr/86Sr ranging from 0.706893 to 0.711632 and 87Rb/86Sr ranging from 0.185 to 0.317. Despite of these differences, data for all the samples — country rocks and impactites — show some positive correlation, if plotted in a Rb/Sr isochron diagram (Fig. 4). The data points scatter along a trend line whose slope corresponds to an age of 1846 ± 160 Ma, with an initial Sr isotope ratio of 0.703 ± 0.002. Interestingly, this age is in good agreement with the “metamorphic age” of ~ 1.8–1.9 Ga that was tentatively derived from our U–Pb dating of zircon grains (Fig. 3a). However, due to the extensive alteration of some of the investigated samples, and because both the metasedimentary rocks and the suevite samples represent a mixture of mineral and rock clasts derived from one or several precursors, it is difficult to consider this age as a reliable “metamorphic age”. An initial 87Sr/86Sr ratio of 0.701 was obtained by Koeberl et al. (1998) for the Bosumtwi country rocks and Ivory Coast tektites. Within these brackets, our results are also in agreement with the initial 87Sr/86Sr ratios for Birimian granitoids, ranging from 0.701 to 0.704 (c.f., Taylor et al., 1992). The investigated metasedimentary basement rocks have εNd values ranging from − 19.7 to − 23.1; these values are similar to those of Birimian sediments (and granitoids) from other parts of Ghana (see Taylor et al., 1992).

The 87Sr/86Sr and 87Rb/86Sr ratios also suggest that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite), and that Ivory Coast tektites, with somewhat higher 87Sr/86Sr and 87Rb/86Sr ratios, are a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than for suevites. These observations, in agreement with conclusions from geochemical studies by Koeberl et al. (1998) and Ferrière et al. (2010), are further supported by the εSr and εNd values (Fig. 5).

Plot of εSr vs. εNd for Bosumtwi crater impactite samples from core LB-08. Our data are compared with values for suevite and country rocks samples from outside the crater rim, as well as with Ivory Coast tektites (data from Shaw and Wasserburg...

Ivory Coast tektites and meta-graywacke samples show a narrow range of εSr and also a relatively narrow range of εNd values. In contrast, the fine-grained metasedimentary rocks (i.e., phyllite and shale) show extremely large variations in both εSr and εNd values (Fig. 5). The suevite samples display similar narrow range of εSr values as observed for Ivory Coast tektites and meta-graywacke samples, but a larger range of εNd values. With the exception of one suevite sample (i.e, sample BCC-5A-64, from Koeberl et al., 1998; see Table 1), suevites and Ivory Coast tektites plot within the range of values of Bosumtwi basement and country rocks. The smaller variation in εNd values, as observed for Ivory Coast tektites, compared to suevite samples, is possibly the result of a smaller variety of different lithologies that were mixed for the production of tektites. In effect, it is now well established that tektites formed by melting of surficial rocks (see, e.g., Koeberl, 1994), whereas suevites result from the mixing of a large amount of target rocks which are excavated during the formation of the crater. Koeberl et al. (1998) suggested that this difference could result also from a better homogenization of the target rock compositions in the formation at higher temperatures of the Ivory Coast tektites compared to the suevites. However, it cannot be totally excluded that part of this difference in the εNd values results from the higher post-impact alteration of suevite samples compared to the Ivory Coast tektites that were quenched and then remained cold. The absence of an overlap in the data ranges for the Ivory Coast tektites and the suevites in Fig. 5 is further evidence that a different mixture of Bosumtwi target rocks formed these two different impactite types. The Ivory Coast tektites tend to plot somewhat closer to the phyllites than to the meta-graywackes (which in turn plot closer to the suevitic breccias), possibly indicating a higher contribution of altered or weathered surficial rocks in the tektites. This is also in agreement with the fact that the rocks exposed at the surface at the time of impact (i.e., the tektite source rocks) were of somewhat different composition (in part because of surface alteration) than those a few tens to hundred meters below the surface, from which the suevites were formed.

6. Conclusions

Single crystal U–Pb zircon ages from suevite and meta-graywacke samples from drill core LB-8A recovered from the central uplift of the Bosumtwi crater yield an upper Concordia intercept age of 2145 ±82 Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana.

Whole-rock Rb–Sr and Sm–Nd isotopic compositions of suevite and basement rocks samples from the same drill core show that the suevite samples are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites, which is thought to consist mainly of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite and shale than the suevite samples. All together, our Rb–Sr whole-rock data combined with previous data yielded a Rb–Sr scatterchron age of 1846 ± 160 Ma, with an initial Sr isotope ratio of 0.703 ± 0.002. However, due to the extensive alteration of some of the samples, this age, which is in good agreement with a possible “metamorphic” age of ~ 1.8–1.9 Ga tentatively derived from our U–Pb dating of zircons, is not considered as a reliable metamorphic age; nonetheless, the general positive correlation of the data pool (n = 29, including literature data) may indicate derivation of the material from a common ancient (Early Proterozoic) source whose Rb–Sr systematics have not basically been changed by the Bosumtwi impact event c. 1.07 Myr ago. In a εSr and εNd plot, data for Ivory Coast tektites and suevites do not overlap (although they fall within the range defined by the country rocks), indicating a different target rock mixture for these two impactite types, in agreement with their current model of formation.

Acknowledgments

This work is supported by the Austrian Science Foundation (FWF), grant P18862-N10, the Austrian Academy of Sciences, and the National Science Foundation of China (NSFC), grant No. 40602023. M. Horschinegg is gratefully acknowledged for her assistance with the Sr–Nd isotopic analyses of the samples. We are grateful to U. and E. Klötzli and to F. Biedermann for discussions and assistance with zircon preparation. F. Brandstätter is acknowledged for assistance with the cathodoluminescence work. This paper is dedicated to the memory of Charlie Schnetzler, a pioneer in the isotopic study of Bosumtwi crater rocks and tektites, who died on the 15th of December 2009, aged 79, from injuries sustained in an earlier car accident.

Shaw H.F., Wasserburg G.J. Age and provenance of the target materials for tektites and possible impactites as inferred from Sm–Nd and Rb–Sr systematics. Earth and Planetary Science Letters. 1982;60:155–177.